1. Field of the Invention
The present invention relates to a stepless transmission for automobiles or other vehicles and, more particularly, to a stepless transmission having a torque converter.
2. Description of Related Art
Some stepless transmissions of this kind have two, or first and second, power flow paths through which engine output is transmitted to a transmission output shaft. When the first power flow path is established, engine output travels from an engine through a torque converter and a forward-reverse change and speed reduction gear to the transmission output shaft. When the second power flow path is established, engine output travels from the engine through a clutch and a stepless transmission gear to the transmission output shaft. In such a stepless transmission, the first power flow path is provided when the vehicle starts, and the second power flow path is provided when the vehicle speeds up and needs no torque multiplication by the torque converter. The provision of two power flow paths which are selectively established enables stepless transmissions to be constructed compactly. Such a stepless transmission is known from, for instance, Japanese patent application No. 1-59,625, entitled "Transmission For Automobiles," filed on Mar. 14, 1989 and published as Japanese Unexamined Patent Publication No. 2-240,444 on Sep. 25, 1990.
Typically, a stepless transmission includes a torque converter and has a forward-reverse change and speed reduction gear and stepless transmission gears. A typical stepless transmission needs an oil pump for operating and lubricating these transmission elements. When considering the necessary amount of oil which must be supplied to the hydraulically controlled elements of the transmission, when the first power flow path, which includes the torque converter, is established, since the stepless transmission gear does not transmit engine torque therethrough, it is not necessary for the stepless transmission to be hydraulically controlled. On the other hand, when the second power flow path, which includes the stepless transmission means, is established, since the torque converter is locked so as to provide a one to one ratio of rotation between its driven and driving members, i.e., a pump and a turbine, and allows no relative rotation between the pump and the turbine, and since the forward-reverse change and speed reduction gear is not unlocked or released, no oil is needed for operating only the torque converter and forward-reverse change and speed reduction gear. Accordingly, in order to apply a sufficient quantity of oil to all hydraulically controlled elements of the stepless transmission and to circulate the oil, a large capacity oil pump is essential. When a large capacity oil pump is used, however, only a part of the discharged oil is effectively utilized; the remaining part of the discharged oil is returned into an oil tank without being used.